An optical modulator, particularly a traveling type optical modulator, using lithium niobate (LiNbO3), lithium tantalate (LiTaO3) or gallium-arsenide (GaAs) for the optical waveguide has excellent properties and may realize a broadband modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent ferroelectric materials having large electro-optic coefficients and can control light within a short optical path. Factors suppressing the modulation speed of the traveling-wave optical modulator include velocity mismatch, dispersion, electrode conductor loss, dielectric loss and mismatch of impedance.
Such type of optical modulator normally includes a substrate, an optical waveguide, modulating electrodes including signal and ground electrodes and a buffer layer, and thus has a relatively complex structure. The dimension of each element has been proposed and variously studied until now.
The assignee filed a Japanese patent publications H10-133, 159A (U.S. Pat. No. 6,219,469) and 2002-169133A, disclosing a traveling wave optical modulator with an optical waveguide substrate having a thinner portion with a thickness of not more than 10 μm where the optical waveguide is formed. It is thereby possible to realize high-speed modulation without forming a buffer layer made of silicon dioxide, and to advantageously reduce the product “Vπ·L” of a driving voltage Vπ and a length “L” of an electrode.
Further, so called multi-media industry has been developed and broad band communication has been increasingly demanded, so that it has been already applied an optical communication system operating at a speed higher than 10 Gb/s. Such system operating at even higher speed is expected. An LN optical modulator is applied as a device for modulating electrical signal of 10 Gb/s or higher (microwave signal) to light.
It was invented a structure for attaining velocity matching of microwave and light wave to realize wide-band modulation of an optical modulator, by lowering the thickness of an optical waveguide substrate. Further, in the structure having the thinner optical waveguide substrate, it is necessary to reduce the thickness of the substrate around its optical waveguide to about 10 micrometers for satisfying the velocity matching condition. The assignee further filed Japanese patent publication 2002-169133A disclosing a two-step groove structure at the back face, for preventing the flattening of the optical mode field pattern and to reduce the transmission loss of light generated due to the surface roughness and damage caused by the processing for reducing the thickness of and forming a groove in the substrate. Further, it is possible to form the two-step groove structure in the substrate after the thickness of the substrate is made thinner and uniform in producing the groove structure. In this case, Japanese patent application 2001-101729 was filed disclosing a structure of providing a reinforcing substrate for maintaining the mechanical strength of the whole device.
The device described in Japanese patent publication H9-211402A has a structure capable of satisfying the velocity matching condition by providing an air layer in the reinforcing substrate. Further, a device described in Japanese patent publication 2001-235714A has an optical waveguide formed on the adhesive face to the supporting body.
According to the devices of Japanese patent publications 2002-169133A and 2001-101729A, however, a groove is formed on the back face of a substrate for modulation and the substrate is then joined with the reinforcing substrate with an adhesive layer made of a material of a low dielectric constant. According to this kind of structure, it has been proved that the temperature drift or DC drift may be made considerable when an excessive load is applied thereon in a reliability test such as thermal shock and temperature cycle tests.